A wide assortment of implantable medical devices (IMDs) are presently known and in commercial use. Such devices include cardiac pacemakers, cardiac defibrillators, cardioverters, neurostimulators, and other devices for delivering electrical signals to a portion of the body and/or receiving signals from the body. Pacemakers, for example, are designed to operate so as to deliver appropriately timed electrical stimulation signals when needed, in order to cause the myocardium to contract or beat, and to sense naturally occurring conduction signals in the patient's heart.
Devices such as pacemakers, whether implantable or temporary external type devices, are part of a system for interacting with the patient. In addition to the pacemaker device, which typically has some form of pulse generator, a pacing system includes one or more leads for delivering generated signals to the heart and for sensing cardiac signals and delivering those sensed signals from the heart back to the pacemaker. As is known, pacemakers can operate in either a unipolar or bipolar mode, and can pace the atria or the ventricles. Unipolar pacing requires a lead having only one distal electrode for positioning in the heart, and utilizes the case, or housing of the implanted device as the other electrode for the pacing and sensing operations. For bipolar pacing and sensing, the lead typically has two electrodes, one disposed substantially at the distal tip end of the lead, and the other spaced somewhat back from the distal end. Each electrode is electrically coupled to a conductive cable or coil, which carries the stimulating current or sensed cardiac signals between the electrodes and the implanted device via a connector.
Combination devices are available for treating cardiac arrhythmias that are capable of delivering electrical shock therapy for cardioverting or defibrillating the heart in addition to cardiac pacing. Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, uses coil electrodes for delivering high-voltage shock therapies. An implantable cardiac lead used in combination with an ICD may be a quadrapolar lead equipped with a tip electrode, a ring electrode, and two coil electrodes. A quadrapolar lead requires four conductors extending the length of the lead body in order to provide electrical connection to each electrode.
In order to perform reliably, cardiac pacing leads need to be positioned and secured at a targeted cardiac tissue site in a stable manner. One common mechanism for securing an electrode position is the use of a rotatable fixation helix. The helix exits the distal end of the lead and can be screwed into the body tissue. The helix itself may serve as an electrode or it may serve exclusively as an anchoring mechanism to locate an electrode mounted on the lead adjacent to a targeted tissue site. The fixation helix may be coupled to a drive shaft that is further connected to a coiled conductor that extends through the lead body as generally described in U.S. Pat. No. 4,106,512 to Bisping et al. A physician rotates the coiled conductor at a proximal end to cause rotation of the fixation helix via the drive shaft. As the helix is rotated in one direction, the helix is secured in the cardiac tissue. Rotation in the opposite direction removes the helix from the tissue to allow for repositioning of the lead at another location.
When using such a lead, it is desirable to prevent the ingress of body fluids into the lead body. Blood or other body fluids entering the lead body can create a pathway for infection, a serious complication for implantable devices. Furthermore, the entrance of blood into the lumen of a lead body can interfere with the insertion of a stylet used for lead positioning during implantation and with the final connection of the lead to an implantable medical device.
Methods for sealing the distal end of the lead body while still allowing a coiled conductor and drive shaft to rotate for advancing or retracting a fixation helix are known. One method is to provide a sealing membrane within the lumen of the distal lead tip. Reference is made to U.S. Pat. No. 4,311,153 issued to Smits. When the helix is advanced, the pointed tip of the fixation helix punctures the sealing membrane, which provides a seal around the fixation helix. When used during implantation, multiple turns of the coil may be required in order to build up torque to overcome the friction encountered when rotating the helix through the membrane. The helix may not advance by the same amount with each turn applied to the coil. Therefore, the extension or retraction of the helix may be somewhat unpredictable. The punctured membrane may not always form a fluid-tight seal around the fixation helix.
Another method for sealing the lumen of a medical lead involves positioning a sealing ring to encircle the drive shaft connected to the fixation helix. This type of seal may be maintained in a desired location by retainers mounted proximal and distal to the seal. Reference is made to U.S. Pat. No. 5,948,015 issued to Hess et al.
Pacemaker systems, as well as other medical devices such as those mentioned above, can utilize a wide variety of lead designs. Many considerations are taken into account when optimizing the design of a lead. For example, minimizing lead size is important since a smaller device is more readily implanted within the cardiac structures or coronary vessels of a patient. Moreover, providing features that make a lead easier to implant and extract allows the clinician to complete the associated surgical procedure more safely and in less time. Finally, an optimized lead design requires a minimum number of parts that may be assembled using techniques that are relatively simple and low cost.
A medical lead having an improved seal against body fluids and which allows precise control over the rate of advancement of a fixation helix is therefore needed. Furthermore, such a seal should not require difficult or costly lead manufacturing techniques. The improved seal preferably is capable of withstanding the high pressures encountered within the heart so that, when the lead is used in conjunction with implantable pacemakers or ICDs, blood does not enter the lead lumen and interfere with the implant procedure or cause infection.